docs($http): add better explanation for error status codes

Add explanation for error status codes in `$http` and mention negative codes normalization.
This alleviates confusion about which status codes will trigger the error callback and what values
to expect in such callbacks.

Fixes #11945

Closes #14459

src/ng/http.js

@@ -450,10 +450,13 @@ function $HttpProvider() {
      *   - **config** – `{Object}` – The configuration object that was used to generate the request.
      *   - **statusText** – `{string}` – HTTP status text of the response.
      *
-     * A response status code between 200 and 299 is considered a success status and
-     * will result in the success callback being called. Note that if the response is a redirect,
-     * XMLHttpRequest will transparently follow it, meaning that the error callback will not be
-     * called for such responses.
+     * A response status code between 200 and 299 is considered a success status and will result in
+     * the success callback being called. Any response status code outside of that range is
+     * considered an error status and will result in the error callback being called.
+     * Also, status codes less than -1 are normalized to zero. -1 usually means the request was
+     * aborted, e.g. using a `config.timeout`.
+     * Note that if the response is a redirect, XMLHttpRequest will transparently follow it, meaning
+     * that the outcome (success or error) will be determined by the final response status code.
      *
      *
      * ## Shortcut methods